1. Field of the Invention
The present invention relates to a method for the local forming of brittle materials and to an apparatus for performing said method. The invention relates particularly, but not exclusively, to the joining of metal sheets by "joggle joining" techniques.
2. Description of the Background Art
Many methods for "joggle joining" of metal sheets are known. Reference may be made, for example, to EP-A-215,449, U.S. Pat. Nos. 4,757,609, and 5,046,228. The disclosures of each of these references are incorporated herein by reference. A common feature of all these methods is that metal sheets are laid flat one upon the other and that material in both sheets is locally joggled, i.e., pressed by a punch into a cavity, care being taken to ensure that the material of the sheets in the region to be joined is tightly clamped together.
This technique has been adopted as an economical production method in many fields, for example in motor vehicle manufacture, in air conditioning and in machine construction when mass production is required.
Known joggle joining tools generally comprise a die defining a cavity into which the sheet material is deformed by means of a punch. An anvil forming the die bottom is disposed opposite the working surface of the punch. Joggle joining tools are known in which the anvil is spring preloaded. U.S. Pat. No. 3,771,216 discloses an arrangement of this kind in which an anvil and spring combination serves as an ejector intended to remove the joint from the die. U.S. Pat. No. 4,584,753 discloses a die where, in the rest position, the anvil projects beyond the edge of the die under spring preloading. The projecting portion of the anvil serves as centering means intended to position a pre-perforated sheet in relation to the die and the punch. In both cases, however, the force produced by the spring is some orders of magnitude less than the forces which are to be applied in accordance with the present invention. The disclosures of both these patents are incorporated herein by reference.
These joining methods can be applied to many metals and plastics materials. However, it has not hitherto been possible to join brittle materials, such as for example certain aluminum alloys, by such techniques, because their strainability is insufficient. As a general rule such brittle materials can undergo non-cutting forming only within a narrow range of material deformations.
It has hitherto not been known what phenomena underlie the empirically established fact that brittle fractures do not occur in forming carried out under an additional pressure load, whereas the same degree of forming without such a pressure load must of course remain within the elastic range of the stress-strain diagram, and is compensated in the forming, in which of course the entire elastic range must necessarily be passed through before plastic deformation occurs.
The object of the invention is to indicate a method which permits greater deformation of brittle materials than was possible hitherto. In particular, the joggle joining of sheets of brittle aluminum alloys is to be made possible.